Can I scale 1 image in animation to bigger and still maintain the visibility of the image in front of it?
So, basically: You can achieve the wanted effect on different ways. You could try it via changing location and so on, but the easiest way is: just scale the images and set key animations.

Go to the start frame, select the image of the e.g. earth. Press "S" and scale it to the wanted start size. Now press "I" and choose "Scaling" from the pop-out-menu. Now go to the next frame you want to define (for example the last one) and repeat the steps. Same you can do for the alien. Just make sure to have a small space between alien and earth for avoiding weird overlaps.

For more information about animating, google a bit ;)

I am not sure whether I understood your last question right (^^) but as scaling just affects the size and not position, overlapping another image by "overscaling" shouldn't happen. Provided there are really only images (that have no depth). Object can overlay each other if the depth becoms too big, logically.

Hope it helped :)
Greets, Jaspa